Underfill or encapsulant compositions for mounting integrated circuit (IC) devices are known, including epoxy and phenolic resin formulations typically incorporating hardeners, theological additives, fillers and the like. Such epoxy resin formulations are known to provide a good combination of thermal, mechanical and electrical properties, along with processing ease and relatively low cost. The filler components of such formulations may represent sixty percent by weight or more thereof, serving to enhance the mechanical strength and reduce the coefficient of thermal expansion (CTE) of the cured material. Such flip-chip assembly techniques using an encapsulant material is shown, for example, in U.S. Pat. No. 5,089,440 to Christie et al, the disclosure of which is incorporated herein by reference.
Such encapsulant formulations are used in mounting so-called flip-chip IC devices to a circuit board. (See, for example, Encapsulants Used in Flip-Chip Packages, D. Suryanarayana et al, IEEE Transactions, Vol. 16, No. 8, December 1993.) Flip-chip processing involves metallurgically bonding an IC device face down to the substrate, that is, with the surface of the IC device from which its electrodes emerge oriented toward the surface of the substrate. Although the metallurgical bonds, formed typically by soldering the IC device electrodes to the substrate such as by Sn--Pb solder for example, or by Au--Au solid diffusion, Au--Sn solid-liquid diffusion or other suitable metallurgical bonding technique, are generally reliable, application of this technology has been somewhat limited due to the potential for inadequate thermal stress durability of the metallurgical bonds during thermal cycling. Thermal stresses are caused by thermal expansion differentials between the substrate, typically glass or other fiber reinforced material such as epoxy or the like, and the silicon or other material of the IC device body. The encapsulant materials are used as a polymeric "underfill" adhesive between the body of the IC device and the substrate to improve the fatigue life of flip-chip assemblies during thermal cycling. Relatively large thermal expansion differentials exist in such flip-chip assemblies with underfill due to the high CTE of known encapsulant formulations. Thermal stresses would be reduced by using a polymeric underfill material having a CTE which matches or more closely approximates that of the metallurgical bonds. In a typical tip-chip assembly, a gold electrode "bump" is used, having a coefficient of thermal expansion approximately 15-17 ppm/.degree.C. Known encapsulant formulations typically exhibit glass transition temperature values in the range of 155.degree.-160.degree. C., with a CTE (below T(g)) of approximately 25-30 ppm/.degree.C. It would be desirable to decrease stresses induced by mismatch of thermal expansion of the materials used in an integrated circuit assembly and increase useful operating temperature of the assembly by employing an underfill composition having lower CTE values and higher glass transition temperature values.
It is an object of the present invention to provide a method of producing an integrated circuit assembly employing a polymeric underfill material having good CTE performance over a large thermal cycling temperature range. Additional objects and aspects of the present invention will be apparent in view of the following disclosure and detailed description of certain preferred embodiments.